Lactate is a metabolite of the anaerobic metabolism of glucose. During this metabolic process, one molecule of glucose is cleaved into two lactate molecules, whereby energy is obtained for the cell via the production of adenosine triphosphate (ATP). As this process takes place without the use of oxygen, it is referred to as anaerobic metabolism. While the former assumptions that the formation of lactate in the body leads to acidosis and, thereby, after the exercises to muscle aches have been largely disproven, the lactate level and, in particular, the lactate threshold, are still of high interest for the assessment of the physical capabilities of a person.
In connection with the assessment of the performance of a person and the effectiveness of training, one of the values generally looked at is the lactate threshold which is also sometimes referred to as the anaerobic threshold. This designates the exercise intensity at which lactate is formed quicker than it can be broken down, and, therefore, accumulates in the blood stream. Since any training for endurance sports, such as long distance running as well as cycling or swimming, leads to the best results if performed under aerobic conditions, the lactate threshold is a good indicator for deciding on the training intensity for endurance sports. Endurance training is widely used in competitive sports but it is also gaining considerable impact in the prevention and rehabilitation of a number of diseases including coronary heart disease, chronic heart failure, diabetes and cancer.
At the moment, the lactate threshold is measured by taking blood samples from a person during a ramp test, whereby the exercise intensity is progressively increased. The individual aerobic threshold, which is widely used as an anchor point for training recommendation can then be determined from these blood samples. The individual aerobic threshold also reflects the so-called maximal lactate steady state (MLSS), which is defined as the highest blood lactate concentration that can be identified as maintaining a steady-state during a prolonged submaximal constant workload. This technology has got the disadvantage that first of all it involves taking blood samples from a person, which is painful and inconvenient. Furthermore, the analysis of the blood either requires laboratory equipment which, therefore, puts the analysis of the lactate threshold out of reach of most people, or is performed by portable devices like glucose measurement devices which are, however, invasive. The analysis of the blood samples in a laboratory takes time so that this technology is not suitable for monitoring the body lactate levels in normal and, in particular, outdoor training scenarios. The disadvantage of an invasive lactate monitoring during a training session is the necessity to interrupt exercising for capillary blood sampling. A non-invasive continuous lactate measurement approach would be an innovative solution and would allow application of lactate monitoring in a broader community.
U.S. Pat. No. 5,757,002 describes a method as well as an apparatus for measuring lactic acid in an organism. The system described in this document is based on the reflectance and/or absorbance of near infrared light. The device described in this document though is large and cumbersome and is not suitable as a monitoring device in the usual endurance sports training scenarios.
US 2005/0192493 describes what is called a non-invasive blood analyzing measuring system with some self-calibration capability. The radiation used for analysis purposes again is infrared radiation, in particular near infrared radiation, and lactate is mentioned as one of the potential analytes. The device described is designed to be used on the earlobe which, again, is undesirable for use in endurance sports training scenarios, as the motion experienced by the device will significantly impair the ability of generating reliable measurements. Furthermore, in view of the applicant's experiences, so far it seems doubtful whether reliable lactate measurements can at all be achieved with the described device. Furthermore, it needs to be noted that the main focus of this document is measuring blood glucose in order to monitor diabetes patients.